Regime forces on Monday strafed rebel-held districts in Aleppo with helicopter gunships and pounded them with shelling on the third day of a pitched battle for Syria's commercial capital.

The fighting has sent some 200,000 civilians fleeing the northern city, according to the UN, which warned of a looming humanitarian catastrophe, while France said it would call an urgent UN Security Council meeting on Syria.

The head of the UN observer mission inside Syria said he was deeply concerned by the violence from both sides in Aleppo. He said he himself had also seen heavy shelling of Homs, Syria's third largest city, during a field visit Sunday.

The army's offensive in Aleppo was focused on Salaheddin district in the southwest, a stronghold of the rebel Free Syrian Army, made up of deserters and armed civilians, said the Syrian Revolution General Commission.

Colonel Abdel Jabbar al-Oqaidi, head of the FSA military council for the city, said that "several quarters of Aleppo are being bombed with MiG (fighter jets) and helicopters." Just outside of Aleppo, rebels seized a strategic checkpoint after a 10-hour battle, an AFP correspondent reported.

"The Anadan checkpoint... was taken this morning at 5:00 (0200 GMT) after 10 hours of fighting," said General Ferzat Abdel Nasser, a rebel officer who deserted the Syrian army a month ago.

By securing the checkpoint, about five kilometres (3.8 miles) northwest of Aleppo, the rebels now control a direct route between the Turkish border and the commercial capital.

The AFP correspondent said that the rebels captured seven tanks and armoured vehicles, and destroyed an eighth vehicle.

General Abdel Nasser told AFP that six soldiers were killed and 25 taken prisoner, adding that four of his own men died.

The Syrian army claimed to have overrun part of Salaheddin during the night, but the advance was denied by the FSA's Colonel Oqaidi.

"The Syrian army took control of part of Salaheddin district and continues its offensive," a security source in Damascus told AFP.

However, Oqaidi insisted government troops had "not progressed one metre (yard)." Since the launch on Saturday of the regime assault on Aleppo, the FSA "has already repelled three offensives" against Salaheddin, he said, adding that the rebels controlled "between 35 to 40 percent of Aleppo." UN mission chief Lieutenant General Babacar Gaye said he was "deeply concerned about the ongoing violence from both sides in Aleppo." "My observers there have reported an upsurge in the violence, with helicopters, tanks and artillery being used," the Senegalese general said.

"It is imperative that both sides respect international humanitarian law and protect civilians." Gaye said that during his trip to Homs province on Sunday -- his first field visit since taking over the UN mission earlier this month -- he had seen shelling in Homs and extensive damage in the battleground town of Rastan.

"There were damaged tanks left on the side of the streets, public infrastructure, such as bridges, was destroyed, and homes on the main roads inside the town were largely damaged," he said.

200,000 flee Monday's violence followed a bloody day in which 125 were killed across Syria -- 46 civilians, 45 soldiers and 34 rebels -- according to the Britain-based Syrian Observatory for Human Rights.

UN humanitarian chief Valerie Amos said in a statement that an estimated 200,000 people had fled from Aleppo in two days and an unknown number were still trapped in the city.

Amos said that many people in Aleppo had sought shelter in schools and other public buildings. "They urgently need food, mattresses and blankets, hygiene supplies and drinking water," she said.

US Defence Secretary Leon Panetta warned Syrian President Bashar al-Assad that the assault on his own population in Aleppo would be a nail in his coffin.

"It's pretty clear that Aleppo is another tragic example of the kind of indiscriminate violence that the Assad regime has committed against its own people," Panetta told reporters on a military plane en route to Tunisia.

"And in many ways, if they continue this kind of tragic attack on their own people in Aleppo, I think ultimately it will be a nail in Assad's coffin," he said.

French Foreign Minister Laurent Fabius said in Paris on Monday that France, which is taking over the UN Security Council's rotating presidency in August, will call an emergency ministerial meeting on Syria.

Fabius told French radio station RTL he would chair the meeting himself and that it had to be held urgently to stop Assad's regime carrying out massacres in Syria.

More than 20,000 people have been killed in Syria since the anti-Assad revolt began in March 2011, according to the Observatory. There is no way to independently verify this figure, while the UN has stopped keeping count.